The number of babies born exposed to addictive drugs while in the womb is increasing, a new study suggests.

Between 2000 and 2009, the incidence of neonatal abstinence syndrome — a disorder that occurs in babies exposed to illegal or prescription drugs during pregnancy — increased nearly threefold in the United States, the study found.

The rate of mothers using opiates such as heroin, morphine, codeine and Oxycontin at the time of delivery increased fivefold over that same period.

Neonatal abstinence syndrome most commonly occurs in newborns exposed to opiates while in the womb. The condition can bring increased irritability, tremors, seizures and respiratory distress.

During the study period, the average cost of treating babies with neonatal abstinence syndrome (NAS) increased 35 percent, and the length of time these babies stayed at the hospital remained about the same (16 days, on average).

"The increasing incidence of NAS and its related health care expenditures call for increased public health measures to reduce" exposure to opiates before birth, the researchers write, in a paper published online on April 30 in the Journal of the American Medical Association.

In addition, standardizing the care these babies receive, and developing new treatments for the condition may better treat babies with NAS symptoms and reduce the length of their hospital stays, the researchers said.

Researchers from the University of Michigan Health System analyzed information from two national databases — one for children and one for adults — each containing information from about 7 million people released from the hospital.

Between 2000 and 2009, the rate of newborns diagnosed yearly with NAS increased from 1.20 per 1,000 births to 3.39 per 1,000 births. Also over this period, the number of mothers using or dependent on opiates increased from 1.19 per 1,000 births to 5.63 per 1,000 hospital births per year.

Average hospital charges for newborns diagnosed with NAS increased from $39,400 to $53,400, the researchers said.

In 2009, about 13,500 infants were diagnosed with NAS, an estimate that equates to about one infant born per hour with the condition.

Treating mothers who use opiates with a drug called buprenorphine may have advantages over the traditional treatment, which involves use of methadone, the researchers noted, pointing to a 2010 study that showed that newborns whose mothers were treated with buprenorphine required 89 percent less morphine, and spent 43 percent less time in the hospital.